ch 8 cellular mfg

Cellular Manufacturing 1

Cellular ManufacturingAn efficient way to organize your small-quantity-multiple-product manufacturing


Group Technology (GT) Group technology (GT) is a concept that seeks to take

advantage of the design and processing similarities among the parts to be produced. The term “group technology” first was used in 1959, but not until the use of interactive computers became widespread in the 1970s did this technology develop significantly.

Group technology becomes especially attractive because of the ever-greater variety of products available to consumers, which are often produced in batches. Since nearly 75% of manufacturing today is batch production, improving the efficiency of batch production becomes important. The traditional product flow in batch manufacturing, the process-oriented layout, creates large amount of transportation and WIP. For the small-quantity-multiple-product manufacturing, the things will get worse. Such an arrangement is not efficient, because it wastes time and effort. A more efficient product flow line to take advantage of group technology is the product-oriented layout or to form a manufacturing cell.


Work cell layout1. Process-oriented layouts:You collect all like machines together and bring all parts to them.2. Product-oriented layouts:You place machines where they are needed to eliminate excessive moving.

Skipping over machines and backtracking will result from process layouts and must be discouraged because it adds costs without adding to the value (muda).

When many parts are fabricated in one group of machines (called a process layout), jumping around may be necessary, but we want to minimize this jumping, skipping, and backtracking.


Group technology

Part similarity


Group Technology

Group technology takes advantage of similarity in parts or features in a group or family of parts so that these parts can be processed as a group.

Part family

Machine group


Advantages of GT

It makes possible the standardization of part design and the minimization of design duplication. New part designs can be developed using previously used designs.

Data that reflect the experience of the designer and the manufacturing process planner are stored in the database. Thus, a new and less experienced engineer quickly can benefit from that experience by retrieving any of the previous designs and process plans.

Manufacturing costs can be estimated more easily, and the relevant statistics on materials, processes, number of parts produced, and other factors can be obtained more easily.


Advantages of GT

Process plans are standardized and scheduled more efficiently, orders are grouped for more efficient production, and machine utilization is improved. Setup times are reduced, and parts are produced more efficiently and with better and more consistent product quality. Similar tools, fixtures, and machinery are shared in the production of a family of parts. Programming for NC is automated more fully.

• With the implementation of CAD/CAM, cellular manufacturing, and CIM (computer integrated manufacturing), group technology is capable of greatly improving the productivity and reducing the costs in batch production—approaching the benefits of product-oriented layout.


Classification and Coding of Parts

In group technology, parts are identified and grouped into families by classification and coding (C/C) systems. This process is a critical and complex first step and is done according to the part’s design attributes and manufacturing attributes.



Design attributes:• External and internal shapes and dimensions• Aspect ratios (such as length-to-width or

length-to-diameter)• Dimensional tolerances• Surface finish• Part functions



Manufacturing attributes :• Primary processes used• Secondary and finishing processes used• Dimensional tolerances and surface finish• Sequence of operations performed• Tools, dies, fixtures, and machinery used• Production quantity and production rate



Coding can be time consuming, and considerable experience is required. The coding can be done simply by viewing the shapes of the parts in a generic way and then classifying the parts accordingly (such as parts having rotational symmetry, parts having rectilinear shape, and parts having large surface-to-thickness ratios).

The code for parts can be based on a company’s own system of coding.

The three basic types of coding systems are:The Opitz SystemThe MultiClass SystemThe KK-3 System



The Opitz SystemThe Opitz system was developed in the 1960s in Germany by H. Opitz (1905-1977), and was the first comprehensive coding system presented. The basic code consists of nine digits (12345 6789) representing design and manufacturing data. Four additional codes (ABCD) may be used to identify the type and sequence of production operations.

This system has two drawbacks: (a) it is possible to have different codes for parts that have similar manufacturing attributes, and (b) a number of parts with different shapes can have the same code.



The multiClass system It was developed to help automate and standardize several design, production, and management functions and involves up to 30 digits.

The KK-3 system It is a general-purpose system for parts that are to be machined or ground. It uses a 21-digit decimal system. This code is much greater in length than the two previous systems described, but it classifies dimensions and dimensional ratios, such as the length-to-diameter of the part. The structure of a KK-3 system for rotational components is shown in Fig. 38.17.


The structure of a KK-3 system for rotational components


Cell Formation Approaches

Efficient work flow can result from grouping machines logically so that material handling and setup can be minimized. Parts can be grouped so that the same tooling and fixtures can be used. When this occurs, a major reduction in setup results. Machines can also be grouped so that the amount of handling between machining operations also can be minimized.

Part family

Machine group


Cell Formation ApproachesRank-Order Cluster Algorithm

There are several cell formation approaches available. Here, we focus on one of them, the Rank-Order Cluster Algorithm

The left diagonal block shows that two cells are finally formed:C1: Machines: M1 and M2 Parts: 2, 4, and 6C2: Machines: M3 and M4 Parts: 1, 3, and 5


Cell Formation ApproachesRank-Order Cluster AlgorithmExample: Consider a 8-part-and-6-machine problem shown in the following table, form the part family and machine group.



Step 1:We assign column 8 place value 1, column 7 place value 2, column 6 place value 4, and so on. Row A receives a value of 128 + 64 + 8 = 200 for its 1’s in the first, second, and fifth columns. Evaluating all rows produces the values shown.



Step 2: Rank the row in order of decreasing decimal weight values.

The rows are reordered to A, B. C, D, E. F.



Step 3: Repeat steps 1 for each column by assign the value to each row with bottom row’s value 1, 2nd bottom row 2, and so on so for.

This produces the following result:

And reorder the column in decreasing values from left to right, the new ordering is 3,1,2,4,5,6,7,8 (the table is shown in next slide).



Step 4: Repeat steps 1, 2, and 3, finally, the result is given by:

4

Next, repeat step 1 for further ordering. However, the row ordering for the repeat step is unchanged and we stop.

4

Rank-Order Cluster Algorithm



Homework (lab) Problem: Consider a 6-part-and-9-machine problem shown in the following table, form the part family and machine group.

Rank-Order Cluster Algorithm


Work cell Layout


Work cell Layout

The lean production, linked-cell system is the newest manufacturing design.


Work cell Layout


Manufacturing cells produce parts one at a time using standing and walking workers

Work cell Layout


Interim-cell design example

A manufacturing cell produces a family of 4 parts, in this case, 4 pinions.



The part family can be manufactured in the lean-production cells shown in following figures operated by one to three workers.

Left figure shows a one-man operated cell.

The interim-cell: the cell can be operated by one, or multiple workers.



This cell is a less-than-full-capacity design that can be quickly modified for different parts in the product family, and to increase output by adding workers.

Left figure shows a two-man operated cell.



These workers are multifunctional and multiprocessers.

Left figure shows a three-man operated cell.


Cycle time is calculated as below


CT = (MT×O) + (WT×WC)

where CT = cycle time, minutesMT = worker-manual time, minutesO = number of operationsWC= number of walk cycles, or



CT = 1/PR

where PR = production rate, parts/hour.

Throughput time can be calculated as:TT = CT × C

where TT = throughput time, minutesC= number of cycles that the part was in the cell


Cycle time in this example:


CT = (MT×O) + (WT×WC)

= 0.25 minutes × 8 +3 sec. × 8 = 144 sec./part = 2.4 min./part

Number of parts produced = (3,600sec./hour) / (72 sec./part) = 25 parts/hour

Throughput time = (144sec./part) × (8 transfers) = 1152 sec./part or 19.2 min./part



Cycle time min/part

# of parts produced per hour

Throughput time min./part

One-man manufacturing cell

2.4 25 19.2

Two-man manufacturing cell

1.2 50 12 approx.

3-man manufacturing cell

0.9 67 10.8


U-Shape Example


Work cell Layout

ch 8 cellular mfg

Documents

batch manufacturing

manufacturing cell

manufacturing costs

manufacturing process

group of machines

advantage of group technology

term group technology

family of parts